Answer:
B. Sun.
Step-by-step explanation:
The carbon cycle can be defined as the cycle in which carbon atoms travel from the atmosphere to earth and back to the atmosphere. As carbon is the source through which all life on earth exists.
The source of the carbon cycle is solar energy. During the process of photosynthesis, it is the sun that plays an integral role in taking carbon dioxide from plants.
Therefore, option B is the correct answer.